1. Field of the Invention
The present invention generally relates to a filmless intra-oral dental device for detecting or sensing x-rays. More particularly, the present invention relates to a flexible electronic x-ray sensing device that fits inside a patient's mouth.
2. Related Art
Dentists and oral surgeons typically use x-ray radiation to obtain images of intra-oral regions, which aid in diagnosis and treatment of dental disorders. Most conventional x-ray detection techniques use photosensitive film (“x-ray film”) to register an image. For example, in conventional x-ray detection, a film cartridge is inserted in a patient's mouth and, when the cartridge is exposed to x-rays, the x-ray film is exposed and captures an image of an intra-oral region, such as the patient's teeth and/or gums. The x-ray film subsequently undergoes chemical development to make the image on the exposed x-ray film readily visible.
In digital or electronic dental radiography, an electronic detector or sensor is utilized in place of x-ray film, and the sensor converts x-rays into an electrical signal. This type of radiography offers a variety of advantages over film-based x-ray techniques. Firstly, electronic sensors are more sensitive to x-rays than is x-ray film. This allows the dosage or quantity of x-ray radiation that a patient receives, in order to obtain an intra-oral image, to be reduced by as much as 90%. Secondly, the image can be generated by a computer instantaneously, thus eliminating the time-consuming development process and the use of potentially harmful chemicals. Thirdly, digital images can be stored as an electronic file in a computer-readable memory, which enables them to be easily sent to specialists for consultation, such as via the Internet.
A conventional electronic x-ray sensor unit typically is enclosed in a hard, rigid material, which houses and protects a charge-coupled device (CCD) or a complementary metal-oxide-semiconductor (CMOS) imaging array. An example or a CMOS Active Pixel Sensor (APS) array is set forth in U.S. Pat. No. 5,912,942 to Schick et al. (“the Schick '942 patent”), which is assigned to the assignee of the present invention and incorporated herein by reference. The sensors described therein are excellent for their intended applications, in terms of image quality, power consumption, and other characteristics. However, the sensors of the '942 patent and other conventional sensors utilize single-crystal silicon-based technologies, which produce fragile and rigid imaging devices, thus necessitating the use of rigid housing materials. Therefore, conventional sensor units usually are non-compliant or unbendable, which may cause discomfort for certain patients and limit a dental practitioner's ability to correctly position such a sensor unit in a patient's mouth. In contrast, even though film cartridges may also be uncomfortable, as they often are made from stiff cardboard, when a patient bites down for an x-ray “shot” or exposure, the film cartridge can bend a bit in response to the biting action, thus providing the patient with some amount of relief from the discomfort.
Another problem with current electronic sensing systems is their high cost. Solid-state devices such as CCDs have imperfect manufacturing yields and require nearly perfect charge generation efficiency to achieve a good image comparable in quality those achievable with x-ray film. Usually, CCDs are manufactured in boutique foundries and often have distinct manufacturing requirements that are unlike those of typical semiconductor processes. CMOS APS technology is an alternative solid-state imaging technology that takes advantage of the comparatively less expensive CMOS manufacturing processes, which have been optimized for the high-volume computer-chip industry. However, CCDs and CMOS imaging devices for x-ray sensor applications utilize extremely sophisticated equipment and processing procedures, and these imaging devices are inherently expensive to develop and manufacture.
Thin-film transistor (TFT) technology has gained interest from the display industry for its capabilities in producing flat-panel displays, among other things. Conventional TFT displays are manufactured onto rigid glass substrates in a process that involves baking the glass substrates at temperatures exceeding 600° C. This technology also is utilized to manufacture TFT-based amorphous-silicon detectors for the medical imaging industry. The conventional TFT process, however, is much too hot to be used with plastics, and CMOS and CCD processes are similarly unsuitable.
Recently, significant advances have been made in the development of flexible, plastic substrates for the display industry. With these advances, processing techniques have been developed that are compatible with such substrates. For example, U.S. Pat. No. 6,808,972 to Sirringhaus et al. (“the Sirringhaus '972 patent”), which is incorporated herein by reference, describes a solution-based method for forming polymeric TFTs on flexible, plastic substrates. Also, in an article entitled “Printed active-matrix TFT arrays for x-ray imaging” (Proceedings of SPIE Vol. 5745, pp. 7-17, Bellingham, Wash., 2005) by Street et al. (“the Street article”), which is incorporated herein by reference, a jet-printing method is described that is used for forming TFTs on flexible substrates, for use as active-matrix backplanes in medical imaging systems. The Street article discusses the fabrication of an amorphous silicon p-i-n sensor that includes a photodiode layer for x-ray imaging applications.
Dental sensors usually require a variety of component circuitry in order to operate. For example, as schematically illustrated in FIG. 1 and discussed in U.S. Pat. No. 6,134,298 to Schick et al. (“Schick '298 patent”), which is assigned to the assignee of the present invention and incorporated herein by reference, a dental sensing system 10 includes an electronic sensor unit 1 and a remote board 2 connected via a wired interface 3 to a computer 4 through a Universal Serial Bus (USB) port 4a. That is, the dental sensing system 10 includes not only an imaging portion (i.e., the electronic sensor 1), but also includes electronic circuitry to run the imaging portion and to convey information from the imaging portion to an image processing unit (i.e., the computer 4).
More specifically, depending upon the particular design of the electronic sensor unit 1 (e.g., CCD, CMOS APS array, and the like), the sensor unit 1 may include electronic circuitry for generating biasing voltages used to operate the sensing devices, as well as circuitry for generating clocking signals and readout circuitry used to read out image data from the sensing devices. Preferably, the sensor unit 1 includes interface electronics 1a for communicating with, for instance, the remote board 2. Optionally, the sensor unit 1 may include a memory and conditioning circuitry for conditioning a sensed signal (i.e., the image data). For example, U.S. Patent Application Publication No. 2004/0066898 to Schick et al. (“the Schick '898 application”), which is assigned to the assignee of the present invention and incorporated herein by reference, describes a wireless sensor that transmits data for an entire full-mouth series of images, from inside the mouth. For such a wireless sensor, RF transmission circuitry as well as a battery may need to be included within the packaging of the sensor.
U.S. Patent Application Publication No. 2003/0031296 to Hoheisel (“the Hoheisel '296 application”), which is incorporated herein by reference, purportedly describes an x-ray detector that includes a flexible housing, a flexible substrate, a matrix of TFTs, and a flexible x-ray converter. However, as pointed out in paragraph [0024] of the Hoheisel '296 application, “the drive circuits, which are usually composed of crystalline silicon, are not bendable.” The Hoheisel '296 application then suggests that it would be advantageous to “secure these circuits to small, rigid circuit boards and to electrically conductively connect these circuit boards to the detector substrate 11 with flexible interconnects.” It is alternatively suggested in the Hoheisel '296 application to glue the circuits onto the substrate 11 “with a soft adhesive that can compensate for the bending.” The schemes proposed in the Hoheisel '296 application, however, may not be possible if the sensor components are large, or undergo a substantial amount of strain.
In U.S. Patent Application Publication No. 2004/0016886 to Ringermacher et al. (“the Ringermacher '886 application”), which is incorporated herein by reference, a flexible imager is described, which includes a flexible substrate. The Ringermacher '886 application also describes, at paragraph [0021], a read and reset circuit 210 that is “electrically coupled to photosensor array 110 to receive the electrical signals generated in response to incident radiation 75.” Presumably, the electrical coupling is a wire connection to a remote module housing the array, as shown in FIG. 1 of the Ringermacher '886 application. This strategy, however, may be unsuitable for a sensor whose entire electronic packaging must fit within a patient's mouth. Moreover, this strategy may be unsuitable if the sensor is to operate as a wireless sensor.
A further consideration for digital x-ray sensors is that they typically include a variety of nonflexible imaging layers that serve to enable the effective capture of x-ray photons. In one approach, a digital x-ray sensor receives a signal indirectly through an intermediary portion that converts x-ray photons impinging thereon to visible light photons, which are detected by a detection portion and provide the electrical imaging signal. The intermediary portion commonly includes a scintillator and a rigid fiber-optic plate (“FOP”) usually made from glass. As described in the Schick '942 patent, for example, the glass FOP is positioned between the scintillator and the detection portion, thus allowing the converted visible light to pass onto the detection portion but attenuating unconverted x-rays. Without the FOP, any unconverted x-rays that pass through the scintillator can be received by the detection portion and registered as noise.
An alternative approach to digital x-ray imaging, discussed in U.S. Pat. No. 5,886,353 to Spivey et al. (“the Spivey '353 patent”) and incorporated herein by reference, foregoes use of a scintillator and an FOP. Instead, a photoconductor directly converts incident x-rays to charge carriers, which are collected by proximate capacitive nodes. This approach, however, utilizes conventional semiconductor processing techniques and, therefore, is unsuitable for use with plastic materials.
As evident from the above discussion, a number of technical hurdles limit the development of a flexible intra-oral dental sensor.